A motor control center, also referred to as an MCC, is generally a cabinet-like structure that is divided into modular sections designed to accept a plurality of component motor control center units, also referred to as MCC units. An MCC “unit” is generally a drawer-like container of electrical components, such as circuit breakers, motor starters, relays, power meters, programmable logic controllers, and other control circuit components used for electrical power distribution in industrial and commercial applications. The MCC unit slides into an MCC section and plugs into a common power bus running vertically along the back side of the MCC section. The MCC unit includes a connector, also referred to as a stab assembly or stabs, arranged at the back side of the unit, to plug into the power bus. The stab assembly is movable within the MCC unit to contact or disconnect from the power busses, for those MCC units that have a closed door operation. When an MCC unit is installed, serviced, or removed from an MCC section, it is of utmost importance to prevent the operator from being exposed to electrical hazards. In existing MCC units, an operator accessible unit on/off handle on the front of the MCC unit enables a circuit breaker within the unit to be switched off before the MCC unit is installed in or removed from the MCC section.
A unit interlock blocks inserting the MCC unit into the motor control center section, without first actuating the connect/disconnect handle for the connect/disconnect mechanism to retract the stabs to prevent contact with a power bus. To insert the MCC unit into the MCC section, the operator slides the MCC unit into an available opening in the section, so that the back wall of the MCC unit is close to the common power bus running along the back side of the section. In a closed door operation, the operator then actuates the connect/disconnect handle for the connect/disconnect mechanism to connect the stabs of the MCC unit to the power bus.
Pollution-induced tracking is a phenomenon that gradually produces a conducting path on the surface of materials as a result of surface wetting and contamination. Contamination of electrical contacts may cause tracking and tracking may lead to arcing. When inserting or removing an MCC unit in an MCC section, it is desirable that the contact area with the bus, in and around the stab, is clean and free of localized surface contamination, to minimize arcing. Industrial environments are typically exposed to pollution such as dust, salt, humidity, and atmospheric chemical agents. The pollution-induced tracking phenomenon may contribute to arc flashes in the air gap between the conductors, when advancing the stabs to or retracting the stabs from the power bus.